How to implement subscriptions in graphql.js? - graphql

Question

This is a pretty simple question.
How to implement subscriptions in graphql?
I'm asking specifically for when using graphql.js constructors like below ?
I could not find a clean/simple implementation.
There is another question here, but it deals with relay.js - i don't want to unnecessarily increase the nr of external dependencies in my app.
What i have:
module.exports = function (database){
return new GraphQLSchema(
{ query: RootQuery(database)
, mutation: RootMutation(database)
, subscription: RootSubscription(database) -- i can see this in graphiql - see below
}
);
}
function RootSubscription(database){
return new GraphQLObjectType(
{ name: "RootSubscriptionType"
, fields:
{ getCounterEvery2Seconds:
{ type: new GraphQLNonNull(GraphQLInt)
, args :
{ id: { type: GraphQLString }
}
, subscribe(parent, args, context){
// this subscribe function is never called .. why?
const iterator = simpleIterator()
return iterator
}
}
}
}
)
}
I learned that i need a subscribe() which must return an iterator from this github issue.
And here is a simple async iterator. All this iterator does - is to increase and return the counter every 2 seconds. When it reaches 10 it stops.
function simpleIterator(){
return {
[ Symbol.asyncIterator ]: () => {
let i = 0
return {
next: async function(){
i++
await delay(2000)
if(i > 10){
return { done: true }
}
return {
value: i,
done: false
}
}
}
}
}
}
When i run the graphiql subscription, it returns null for some reason:
I'm piecing together code from multiple sources - wasting time and hacking it basically. Can you help me figure this one out?
Subscriptions are such a big feature, where are they properly documented? Where is that snippet of code which you just copy paste - like queries are for example - look here.
Also, i can't use an example where the schema is separate - as a string/from a file. I already created my schema as javascript constructors. Now since im trying to add subscriptions i can't just move back to using a schema as a string. Requires rewriting the entire project. Or can i actually have both? Thanks :)